Day Month Year DropDownLists - Populate three month prior date when Dete is Selected in ASP.Net

Here is modified Code in C#

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            this.PopulateYears(2000, ddlStartYear);
            this.PopulateMonths(1, ddlStartMonth);
            this.PopulateDays(1, ddlStartDay);
        }
    }


    protected void ddlStartYear_SelectedIndexChanged(object sender, EventArgs e)
    {
        PopulateYears(int.Parse(this.ddlStartYear.SelectedItem.Value), ddlEndYear);
    }

    protected void ddlStartMonth_SelectedIndexChanged(object sender, EventArgs e)
    {
        this.PopulateMonths(int.Parse(this.ddlStartMonth.SelectedItem.Value), this.ddlEndMonth);
        this.PopulateDays(1, this.ddlStartDay);
    }

    protected void ddlStartDay_SelectedIndexChanged(object sender, EventArgs e)
    {
        this.PopulateDays(int.Parse(this.ddlStartDay.SelectedItem.Value), this.ddlEndDay);
    }


    private void PopulateYears(int startValue, DropDownList ddl)
    {
        int finalYear;
        if (ddl == this.ddlEndYear)
        {
            finalYear = startValue + 2;
        }
        else
        {
            finalYear = DateTime.Today.Year;
        }

        List<ListItem> data = new List<ListItem>();
        for (int startingValue = startValue; finalYear >= startingValue; startingValue++)
        {
            data.Add(new ListItem { Text = startingValue.ToString(), Value = startingValue.ToString() });
        }
        ddl.DataSource = data;
        ddl.DataBind();
    }

    private void PopulateMonths(int startMotnth, DropDownList ddl)
    {
        int finalMonth;
        if (ddl == this.ddlEndMonth)
        {
            finalMonth = startMotnth + 2; ;
        }
        else
        {
            finalMonth = 12;
        }
        List<ListItem> months = new List<ListItem>();
        for (int startingValue = startMotnth; startingValue <= finalMonth; startingValue++)
        {
            string monthName = this.GetMonthName(startingValue);
            months.Add(new ListItem { Text = monthName, Value = startingValue.ToString() });
        }
        ddl.DataSource = months;
        ddl.DataTextField = "Text";
        ddl.DataValueField = "Value";
        ddl.DataBind();
    }


    private void PopulateDays(int startDay, DropDownList ddl)
    {
        int finalDays;

        finalDays = DateTime.DaysInMonth(int.Parse(this.ddlStartYear.SelectedItem.Value), int.Parse(this.ddlStartMonth.SelectedItem.Value));
        List<ListItem> days = new List<ListItem>();
        for (int startingValue = startDay; startingValue <= finalDays; startingValue++)
        {
            days.Add(new ListItem { Text = startingValue.ToString(), Value = startingValue.ToString() });
        }
        ddl.DataSource = days;
        ddl.DataBind();
        if (ddl == this.ddlEndDay)
        {
            ddl.Items.FindByValue(this.ddlStartDay.SelectedItem.Value).Selected = true;
        }
    }

    private string GetMonthName(int monthValue)
    {
        return (new DateTime(0001, monthValue, 01)).ToString("MMMM");
    }